The local environment of Eu2+ (4 f7, S=7 2) in Ca1-x Eux B6 (0.003≤x≤1.00) is investigated by means of electron spin resonance (ESR). For x 0.003 the spectra show resolved fine and hyperfine structures due to the cubic crystal electric field and nuclear hyperfine field, respectively. The resonances have Lorentzian line shape, indicating an insulating environment for the Eu2+ ions. For 0.003 x 0.07, as x increases, the ESR lines broaden due to local distortions caused by the Eu Ca ions substitution. For 0.07 x 0.30, the lines broaden further and the spectra gradually change from Lorentzian to Dysonian resonances, suggesting a coexistence of both insulating and metallic environments for the Eu2+ ions. In contrast to Ca1-x Gdx B6, the fine structure is still observable up to x≈0.15. For x 0.30 the fine and hyperfine structures are no longer observed, the line width increases, and the line shape is purely Dysonian, anticipating the semimetallic character of EuB6. This broadening is attributed to a spin-flip scattering relaxation process due to the exchange interaction between conduction and Eu2+ 4f electrons. High-field ESR measurements for x 0.15 reveal smaller and anisotropic linewidths, which are attributed to magnetic polarons and Fermi surface effects, respectively. © 2005 The American Physical Society.